1. Field of the Invention
The present invention relates to an integrated tunable laser structure.
Widely tunable semiconductor lasers are key devices for high capacity wavelength division multiplexed (WDM) transmission and photonic switching systems and offer several advantages to increase the capacity, functionality and flexibility of WDM networks. The invention deals with semiconductor lasers and, more particularly, to configurations thereof that provides wavelength tunability. The basic physical mechanism underlying the electronic wavelength tuning is the refractive index control by exploiting carrier injection (plasma effect), well known in the art.
2. Discussion of the Background Art
A large range tunable laser diode is achieved by monolithic integration of a gain section and also an optical filter with electronically adjustable transmission within a laser resonator so that the optical filter is providing a mode filtering mechanism but also the tuning mechanism. The filtering function can be achieved by an optical filter, which can be based on different filter mechanism, namely interferometric filtering, co-directional coupling and contra-directional coupling.
The principal approach using co-directional coupling for wide wavelength tuning is the usage of laser cavities in which the lasing mode consists of a superposition of co-directionally (i.e. forward) coupled waveguide modes. This co-directional filter (e.g. vertical coupler filter (VCF)) selects only one resonator mode, and the tunability is realized by the wavelength shift of this filter function in order to select another cavity mode. One control current is necessary for the wavelength tuning of laser structures employing only co-directional coupling such as the Vertical Coupler Filter (VCF) laser and the Vertical Mach-Zehnder (VMZ) laser. In this wavelength-tuning scheme the emitted wavelength jumps from one cavity mode to the neighboring one as determined by the maximum of the effective cavity gain.
A laser structure containing a Mach-Zehnder interferometer was described in U.S. Pat. No. 5,511,084 and in European Patent specification EP 0664 587 B1 (inventor M.-C. Amann).
A laser, where the mode selection is performed by utilizing a vertical coupler filter (VCF) was presented by Alferness et al. in xe2x80x9cBroadly Tunable InGaAsP/InP Laser Based on a Vertical Coupler Filter with 57-nm Tuning Rangexe2x80x9d, Appl. Phys. Lett., 1992,60, pp. 3209-3211.
However, the filter bandwidth and the wavelength tuning range are proportional to each other, so that the requirement of keeping the filter bandwidth narrow enough for single-mode operation also limits the total tuning range. This limitation is eased in the case of the Vertical Grating Assisted Co-directional Coupler Laser with Rear Sampled Grating Reflector (GCSR) laser where a Sampled Grating Distributed Bragg Reflector (SG-DBR) filter or a Superstructure Sampled Grating Bragg Reflector (SSG-DBR) filter is combined with a VCF (M.xc3x96beig et al. in xe2x80x9c74-nm Wavelength Tuning Range of an InGaASP/InP Vertical Grating Assisted Co directional Coupler Laser with Rear Sampled Grating Reflectorxe2x80x9d, IEEE Photonics Technology Letter, Vol. 5, No, 7, July 1993. and to Pierre-Jean Rigole et al. in xe2x80x9c114-nm Wavelength Tuning Range of a Vertical Grating Assisted Co directional Couple Laser with Super Structure Grating Distributed Bragg Reflectorxe2x80x9d, IEEE Photonic Technology Letter, 7, 7, July 1995.
The double filtering effect allows single mode operation with high side-mode suppression ratio (SSR) for a wide wavelength tuning range. By integrating an additional phase section, the laser allows tuning in a quasi-continuous tuning mode, where each wavelength within the tuning range can be selected.
From the above major disadvantages of prior art solutions became evident: Different applications need special requirements with respect to wavelength control, side mode suppression ratio and optical output power. An efficient stationary side-mode suppression ratio well above 30 dB and optical output power above 13 dBm are demanded for high performance optical communications. Devices without contra-directional coupling such as the VCF or VMZ laser are hardly suited for applications where a high spectral purity is needed. Additionally, due to the discontinuous tuning schemes of these devices, the emitted wavelength jumps during tuning from one wavelength mode to another one. Therefore, only a specific wavelength that does not necessarily correspond to a desired wavelength grid can be selected. The GCSR laser allows single mode operation for a wide wavelength bandwidth. Due to the fine-tuning section the optical phase can be adjusted enabling the possibility of quasi-continuous tuning. However, in the GCSR laser only about 25% of the laser cavity is covered by the active section. This leads to an insufficient optical output power. Additionally, the longitudinal sectioning into four zones represents a disadvantage of the GCSR laser; this makes manufacturing of these components more difficult and increases the total laser length. This in turn reduces the mode selectivity and also limits the direct modulation bandwidth. Also, internal reflections at the grating of the VCF may influence the laser operation. As a consequence, a more sophisticated resonator structure is required for the single longitudinal mode operation with a strong suppression of side modes and high optical output power.
It is an object of the invention to provide an improved laser structure. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
The laser structure of the invention provides at the same time a large quasi-continuous tuning range and a high spectral selectivity. The tunable laser diode can be made with high optical output power and high side mode suppression ratio. The invention comprises the advantage of a laser structure like the GCSR laser while simultaneously supplying the high optical output power like the VMZ laser. Beside the improved performance the presented laser has a much easier fabrication technology compared to the GCSR laser where a coupler section including a grating and additional the active section has to be longitudinally integrated.